Known from the state of the art are temperature sensors for industrial process technology. Their construction is similar to that of thermal, flow measuring devices, with the difference that conventional thermal, flow measuring devices usually use two temperature sensors, which are embodied as equally as possible and which are arranged, most often, in pin-shaped, metal sleeves, so-called stingers or prongs, which are in thermal contact with the medium flowing through a measuring tube or through the pipeline. Most often, they are immersed in the medium. For industrial application, the two temperature sensors are usually installed in a measuring tube. The temperature sensors can, however, also be mounted directly in the pipeline. One of the two temperature sensors is a so-called active temperature sensor, which is heated by means of a heating unit. The heating unit is either an additional resistance heater, or, when the temperature sensor itself is a resistance element, e.g. an RTD—(Resistance Temperature device) sensor, it is heated by conversion of electrical power, e.g. by a corresponding variation of the heating current. The second temperature sensor is a so-called passive temperature sensor: It measures the temperature of the medium with an as small as possible self-warming by the measurement current.
Until now, mainly RID-elements with helically wound platinum wires have been applied in thermal, flow measuring devices. In the case of thin-film, resistance thermometers (TF-RTDs), conventionally, a meander-shaped, platinum layer is vapor deposited on a substrate. In addition, another, glass layer is applied for protecting the platinum layer. The cross section of the thin-film-resistance thermometer is rectangular, in contrast with the RTD-elements having a round cross section. The heat transfer into the resistance element and/or from the resistance element occurs accordingly via two oppositely lying surfaces, which together make up a large part of the total surface area of a thin-film-resistance thermometer.
Known from U.S. Pat. No. 6,230,560 is a thermal, flow measuring device having two resistors, which are exposed to the flow of the measured medium through a measuring tube. Common to all forms of embodiment is that the two resistors are heated with a heating current and that the temperature of the measured medium is measured with a third resistor. The calculating of the flow of the measured medium through the measuring tube occurs, in such case, by means of the temperatures of the two heated resistors calculated from their resistances and the temperature of the third resistance. The two heated resistors are continuously heated by means of a constant electrical current source. From R=U/I with the heating current I and the measured voltage drop U across the heated resistor, its resistance R can be determined. By means of R=R0+(1+αΔT) there results its temperature T=ΔT+TG, with TG being the temperature of the third, unheated resistor.
There is, however, provided, in one form of embodiment, a voltage source for supplying the heated resistors with the heating current. Then, auxiliary resistors are connected in series with the heated resistors. The resistance of the auxiliary resistors is not changed by the heating current—their resistance is essentially temperature independent. Additionally, electrical current measuring devices are provided, which measure the current heating current via the auxiliary resistors. This is then utilized in calculating the temperatures of the heated resistors, such as already described above.
Disadvantageous is the application of at least three resistors, two heated and one unheated, which are so arranged relative to the lumen of the measuring tube that they are in good thermal contact with the measured medium, in order to measure the flow of the measured medium in the measuring tube.
International published application, WO 2007/063111 A2 discloses a circuit of a thermal, flow measuring device, wherein first and second resistors are arranged in the lumen of the measuring tube. The circuit includes, furthermore, a first electrical current source and a second electrical current source. The first electrical current source delivers a heating current, and the second electrical current source produces a measurement voltage via the resistors. At least one switch between the first resistor and the electrical current sources controls the supplying of the first resistor with the heating current or with the measurement current, in that it connects the first resistor in series either with the first electrical current source or with the second electrical current source. If only the first resistor is heated, then the second resistor is connected via a switch at least at times in series with the second electrical current source. If both resistors are heated, then what has been outlined for the first resistor holds also for the second resistor.
The first resistor is, in such case, heated with a constant amount of heat. The supply of the constant amount of heat is controlled by means of the switch, which switches the first and/or the second resistor in series with the first electrical current source. Disadvantageous in the case of this apparatus is that the first electrical current source does not provide a constant electrical current. It is, instead, a voltage source with constant voltage. The heating power to the first resistor depends, thus, strongly on the length of the heating phase. It is calculated from P=(th/T1)*(U2/R), using the voltage U, the resistance value R, the heating period length to and the measuring period duration T1.